1. Field of the Invention
The present invention relates to an apparatus and a method for processing dissociative compound semiconductors and their single crystals such as GaAS, containing a high vapor pressure component used for making laser elements and IC substrate materials.
2. Technical Background
An example of an apparatus for processing a compound material having a high decomposition pressure is an apparatus shown in FIG. 15 which is disclosed in a Japanese Patent 1,490,669. This apparatus is used to produce a single crystal of a compound semiconductor material, such as GAS containing a high vapor pressure component, by a method known as the Czochralski method (CZ method).
In FIG. 15, the reference numeral 1 refers to a hermetic vessel for pulling a single crystal from a melt. The vessel 1 consists of an upper vessel section 2 and a lower vessel section 3 with a sealing material 5 between the upper and lower vessel sections. A push up rod 6 of the lower section 3 is provided with a stress-moderating device 7 so as to keep the stress imposed on the joint section 4 at a suitable value.
In the interior of the vessel 1 is a crucible 9 supported by a susceptor 8, which is rotated by the bottom rod 10. The vessel 1, which houses all of the above components, is heated by heaters 11a, 11b. A vapor pressure control section 12 is provided on the ceiling section of the upper vessel section 2. The interior wall temperature of the vapor pressure control section 12 is kept at a lower suitable temperature compared with the interior wall temperatures of the vessel 1. The vapor pressure within the vessel 1 is kept at a suitable constant pressure by condensing and adjusting the vapor pressure of the high vapor pressure component, thereby keeping the vapor pressure constant within the vessel 1 and maintaining the stoichiometry of the melt 13 in the crucible 9.
A view rod 14 for observing the growing section of a single crystal 18 is provided through the ceiling section of the vessel 1. The pulling rod 15 and the bottom rod 10 pass through a rotating seal 16 containing a liquid sealant such as B.sub.2 O.sub.3. The entire configuration presented above is housed in an external housing 17.
Next, the method of producing a GAS single crystal using the apparatus described above will be explained. In this case, the high vapor pressure component is As and the other component is Ga.
First, a charge of Ga is placed in the crucible 9, and As feed is placed on the bottom plate section 1a of the vessel 1. After evacuating the entire apparatus, the bottom rod is pushed up to seal the vessel 1.
Next, the interior wall of the vessel 1, excepting the bottom plate section la, is heated by the heater 11a, followed by heating the bottom plate section 1a by the heater 11b, thereby heating the feed of As and subliming the As. Simultaneously, the Ga in the crucible 9 in the vessel 1 is heated to absorb the As vapor and react with the As, thereby synthesizing GAS in the crucible 9.
In this instance, the temperature distribution in the vessel 1 is controlled so that the bottom plate section 1a and the interior wall section of the vapor pressure control section 12 are at lower temperatures compared with those of regions in the vessel 1 so as to prevent As from condensing on the other regions. At the same time, an inert gas is introduced into the external housing 17 to obtain a pressure balance between the interior and exterior atmospheres of the vessel 1.
After completing the synthesizing step of the GAS melt 13 in the crucible 9, a single crystal seed A fixed to the bottom end of the pulling rod 15 is first immersed in the GAS melt, and then the seed A is rotated and pulled up by the pulling rod 15 while the temperature of the heaters 11a and 11b are lowered to grow a single crystal 18 as illustrated in FIG. 15.
The amount of As feed placed on the bottom plate section 1a of the vessel 1 is a sum of: the amount required to synthesize the GAS melt; the amount to fill the interior space of the vessel 1 as a gas at a certain constant pressure after the completion of the GAS melt synthesis; the amount condensed in the vapor pressure control section during the synthesizing and the crystal growing stages; and the amount lost from the vessel 1 during the various stages of production of the single crystal. The vapor pressure control section 12 has a volume capacity sufficient to hold the enough amount of As required to retain solid As until the completion of the production process even after supplementing the amount lost from the vessel 1.
However in the apparatus described above, it is difficult to control the sublimation rate of the As feed during the synthesis stage during which the melt temperature rises rapidly, and the sublimation rate of As becomes higher than the absorption rate of As vapor into the Ga liquid. This causes the As vapor pressure to rise in the vessel 1, and if a pressure balance between the interior and exterior atmospheres of the vessel could not be restored in time to compensate for such a rapid rise in the vapor pressure in the interior of the vessel 1, the sealing between the vessel sections may be destroyed, causing a large amount of As vapor to escape from the vessel 1. Therefore in the above apparatus, it was necessary to strictly control the temperatures of every region of the vessel 1, thus presenting difficulties in automating the process.
This problem was studied extensively by the present inventors, who discovered that in the above apparatus, the problem arose from the difficulty to thermally insulate the As feed material, placed on the bottom plate section 1a, from the parts held at high temperature like the crucible 9, and as the temperature of the crucible 9 rose, so did the temperature of the As feed material.
To lessen the degree of this problem, it is effective to lengthen the vertical dimension of the vessel 1 so as to distance the bottom plate section 1a away from the crucible 9. This approach is unsuitable however, because the vessel has a high thermal conductivity, and to achieve sufficient effect of such thermal isolation, it was necessary to lengthen the vessel 1 considerably, which made the operation of the apparatus inefficient.
The problem described above is not limited to the apparatus for growing single crystal such as that shown in FIG. 15. The same problem arises in compound synthesizing apparatuses and processing apparatuses designed to handle high vapor pressure materials.
The present invention was made in view of the technological problem present in the conventional apparatus as described above, and an objective is to present an apparatus and a method for accurately and reliably performing synthesis of compound materials having high vapor pressure, without sacrificing the operational facility of the apparatus.